Abstract
In IB we introduced the characteristic functions c S for subsets S of a set U and proved (IB2) that the function c S ↦ S is a bijection between KU and P(U). Subsequently it was to be verified (Exercise IB3) that this same bijection made the assignments c S + c T ↦ S + T and c S c T ↦ S ∩ T. We have thereby that (P(U), +, ∩) is “algebra-isomorphic” to the commutative algebra (KU, +, ·), and hence (P(U), +, ∩) is a commutative algebra over the field K. In particular, (P(U), +) is a vector space over K, while (P(U), +, ∩) is a commutative ring; ∅ is the additive identity and U itself is the multiplicative identity. For the present we shall be concerned only with the vector space structure.
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© 1977 Springer-Verlag, New York Inc.
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Graver, J.E., Watkins, M.E. (1977). Algebraic Structures on Finite Sets. In: Combinatorics with Emphasis on the Theory of Graphs. Graduate Texts in Mathematics, vol 54. Springer, New York, NY. https://doi.org/10.1007/978-1-4612-9914-1_2
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DOI: https://doi.org/10.1007/978-1-4612-9914-1_2
Publisher Name: Springer, New York, NY
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